Access violating when pushing into priority queue

I have the following code:

bool custom(pair<int, int>& a, pair<int, int>& b)
{
    return (double)a.first / (double)a.second > (double)b.first / (double)b.second;
}

int reviews(vector<pair<int,int>>& ratings, int t) {

    priority_queue<pair<int, int>, vector<pair<int, int>>, decltype(&custom)> q;
    int numProducts = ratings.size();
    double rating = 0;
    int numRevs = 0;

    // fill up queue and compute initial rating
    for (int i = 0; i < ratings.size(); i++)
    {
        rating += ((double)ratings[i].first / (double)ratings[i].second) / numProducts;
        q.push(ratings[i]);
    }
// other stuff, crashes before this point

    return numRevs;
}

int main()
{
    vector<pair<int, int>> ratings = { {4,4}, {1,2}, {3,6} };
    int t = 80;
    cout << reviews(ratings , t) << endl;
        

}

Which gives the following error the second time q.push() is run: Exception thrown at 0x00000000 in ConsoleApplication1.exe: 0xC0000005: Access violation executing location 0x00000000.

Does anyone know why this is? Thanks

Pass custom method as q's parameter. So, change this:

priority_queue<pair<int, int>, vector<pair<int, int>>, decltype(&custom)> q;

to this:

priority_queue<pair<int, int>, vector<pair<int, int>>, decltype(&custom)> q(custom);

Run it Online

The reason for that is that in std::priority_queue, the third template parameter is:

Compare - A Compare type providing a strict weak ordering.

That means that in your code you had only specified the type of the comparator only (decltype(&custom)), a function pointer. As a result the function object will be default-constructed. Using a default constructed function pointer will invoke undefined behavior. So you need to pass the actual function object, in order for it to be correctly usable.

PS: You might want to take a look in this relevant lambda-using example.